These translations are proving to be challenging as most languages combine it with alone. The difference between the two in my experience is that alone represents being cut off from all connections, personal or otherwise while solitary means just separating oneself from other people.
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English – Solitary

German – einsiedlerisch

Chinese – 独行 “do sheeng”

Spanish – solitario

Japanese –独居“dookyo”
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If anybody else knows of how to say “solitary” in other languages, feel free to share them in the comments and take part in the word of the day challenge.